Chelsea lodged a formal complaint with the Football Association on Wednesday over "inappropriate language" that the club claims referee Mark Clattenburg used against John Obi Mikel during a Premier League match between the Blues and Manchester United.
The Blues decided not to pursue an allegation that Clattenburg also used words understood to have been interpreted as racist toward Juan Mata during Sunday's game following an internal investigation carried out by external legal counsel.
Chelsea submitted signed witness statements from a number of players and staff to the FA, who had already begun their own probe into the matter, according to Press Association.
"Board members began establishing the details of the allegations, speaking to all relevant players and staff," said a Chelsea statement. "After those initial interviews, it was clear that the matter had to be reported. The correct protocol for doing so was to report the incidents to the match delegate, and the club took immediate steps to inform him as soon as he was available. It was not a decision the club took lightly."
The statement said there was "not sufficient evidence to support a second claim" for Clattenburg's alleged words to Mata.
A criminal investigation also began on Tuesday when police formally opened a case into claims against Clattenburg. London's Metropolitan Police said it acted on what it described as a "complaint" made by the Society of Black Lawyers.
"Putting aside claims about Clattenburg (unacceptable if true) when are we as a game (all teams) going to stop players harassing refs?" tweeted former Aston Villa and England team defender Gareth Southgate.
Leeds manager Neil Warnock blasted Chelsea for filing the complaint.
"I'm disgusted at what has gone on. I am on Mark Clattenburg's side," The former QPR boss told talkSPORT. "We ask referees to deal with it and man-manage and that's what he does, Clattenburg. If Chelsea had won that game there would not have been one iota of a complaint."
Chelsea boss Roberto Di Matteo said Wednesday he was not worried about how Chelsea would be refereed after making their complaint against Clattenburg.
"I think the referees should be going into the games with the most confidence that they have," he said. "We're all under pressure here -- the players, the officials, the coaching staff -- to perform. And I don't think there's any prejudice from that point of view."
The Football Association on Monday launched an investigation after Chelsea made an official complaint against the 37-year-old, who has vowed to cooperate fully with their investigation.
"Officers from Hammersmith & Fulham borough are in liaison with Chelsea Football Club and the Football Association," the police statement reads. "At this time, the MPS has not received any complaint from either Chelsea Football Club, or the Football Association.
"We continue to work in partnership with Chelsea Football Club and the Football Association in order to consider any allegation that is made in relation to the reported events."
Clattenburg is yet to respond to the allegations. The Premier League, which authorizes public comment from referees, said he would not make any public statement.
Oriol Romeu has become the first Chelsea player to publicly discuss Sunday's allegations, telling Spanish radio station Cope that Mata told him "there was some problem and he had to stay."
"If there was really a racist comment or something said against a Spanish player, this will be serious," Romeu, an unused substitute in the match, added on Spanish radio station Cope.
The case comes a year after Chelsea captain Terry was investigated by police after allegations he racially abused an opponent during a league match. The defender was cleared in court in July of racially abusing Queens Park Rangers defender Ferdinand, but is serving a four-match ban after being subsequently punished by the FA almost a year after the confrontation.
The FA was forced to halt its case when the police probe into Terry started, but English football's governing body will be keen to avoid another high-profile case remaining unresolved for so long.
Officers have not asked the FA to stop its investigation this time.
For the next week at least, Clattenburg won't be refereeing any matches after the referees' body said the "intense level of scrutiny would detract from the match and be unfair to the clubs and the supporters of both sides."
Clattenburg refereed the men's final at the London Olympics in August and is a FIFA candidate to officiate at the 2014 World Cup in Brazil.
Clattenburg has already filed what is known as an "extraordinary incident report," which deals with a meeting alleged to have taken place in the referees' room after full-time.
Sources told Press Association that Di Matteo, assistant manager Eddie Newton and chief executive Ron Gourlay were all present as Mikel angrily accused Clattenburg of abusing him.
The referee, his assistants and fourth official were stunned by the claims, with Michael McDonough, Simon Long and Michael Jones denying hearing anything of that nature over the officials' microphone link-up, sources said.
The officials' in-game conversations are not recorded but the extraordinary fallout from the weekend's events have led to calls for that to change.
Referees could be canvassed to establish whether they would be in favour of such a move, Press Association reported, which would require the approval of the International Football Association Board, in the same way as goalline technology has.
Information from The Associated Press and Press Association was used in this report.